We have previously demonstrated that a fertilization membrane forms around a sea urchin egg when it is injected with a soluble spermatozoa fraction isosmotic with seawater. We are purifying and characterizing the effective component. It is an acidic, heat- stable protein with an apparent molecular weight 15kD. It appears to be ribosylated. When injected into an egg, it raises cytosolic calcium level while a fertilization membrane is formed.